8.1. Introduction
Tumbling granulators and coaters include discs, drums, pans and a range of similar equipment. In tumbling granulators, particles are set in motion by the tumbling action caused by the balance between gravity and centrifugal forces. These granulators have the following characteristics:
• Product granule size is in the range 2 to 20 mm. Tumbling granulators are not suitable for producing very small granules.
• Tumbling granulators are good for producing high density "balls" or pellets. It is more difficult to produce high porosity agglomerates.
• Tumbling equipment can also be used for coating relatively large (group D) particles.
• Discs and drums generally operate continuously and can be up to 4 m in diameter.
They are capable of very large throughputs (up to 100 tonne/hr) and are therefore extensively used in mineral processing and fertiliser granulation.
In this chapter the operation of different types of tumbling granulators is described focussing on particle motion and mixing, operating variables and scale up. The role of the various granulation processes in tumbling granulators is discussed in some detail.
8.2. Disc Granulators
Figure 8.1 shows a typical disc or pan granulator. It consists of a tilted rotating disc with a rim to hold the tumbling granules. Powder feed are continuously fed to the disc, typically at the edge of the rotating granular bed. Liquid binder is added through a series of single fluid nozzles distributed across the face of the bed (figure 8.2). A coating of feed powder builds up on the disc to form a false base on which the mass rolls. The thickness of this layer is controlled by scrapers or ploughs. The tumbling motion causes segregation and size classification which is central to the disc operation.
Variations on the simple disc design include:
• An outer reroll ring which promotes granule coating and densification without further growth
• Multistepped side walls
• A pan in the form of a truncated cone.
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ranule Hold Up, Mixing and Segregation on the Disc
A key operating parameter for the disc is the critical speed. This is defined as the speed at which a granule is just held stationary on the rim of the disc by centripetal forces alone (see figure 8.3):
D
Nc g 2
2 sin π
= β (8-1)
Discs are typically operated at between 50 and 75% of critical speed with the angle β between 45 and 55°. If the speed is too low, the particle mass will slide against the disc instead of tumbling. If the speed is too high particles are pinned to the rim or may be thrown off the disc. Optimum drum speed is also influenced by the powder flow properties of the feed material.
Figure 8.1. A typical disc granulator (Capes, 1985) 180
TUMBLING GRANULATION
Figure 8.2. Disc granulator details (Sherrington and Oliver, 1981)
Frictional forces between a granule and the disc hold smaller particles longer as they are carried up the disc. The small particles travel higher on the disc and then roll underneath the larger pellets in the eye of the disc (see figure 8.4). Thus the larger well formed granules sit on a bed of new nuclei and ungranulated powder. This process leads to a natural size classification on the disc with only large granules or pellets overflowing the eye of the disc.
As a result of this natural size classification, disc granulators rarely need to recycle undersize and oversize granules which is a distinct advantage over drum granulators. The segregation of granules on the disc also allows the balance of granulation processes to be controlled by adjusting the position of the powder feeder and liquid spray nozzles.
Figure 8.3. Force balance on a granule with the disc operating at critical speed
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Figure 8.4. Granule segregation on a disc granulator illustrating a size classified granular bed sitting on ungranulated feed powder
The total holdup of granules on the disc can be adjusted by varying operating parameters. Hold up will increase with:
• decreasing disc angle at the same fraction of critical speed;
• increasing disc speed: granules are carried higher on the disc and cover a large fraction of the disc area; and
• increasing moisture content: granules are stickier and therefore are carried higher on the disc.
The total hold up determines the average residence time of granules on the disc.
Typical granule residence times are in the range 1 to 2 minutes. However, the mixing patterns (residence time distribution) are also important in determining the product granule properties. The simplest residence time distribution models are continuous stirred tank reactor (CSTR) and the plug flow reactor (PFR). Figure 8.5a shows that a disc granulator lies somewhere between these two extremes. The residence time distribution moves closer to plug flow as the angle of the disc is increased. Figure 8.5b shows a possible two stage model for mixing on the disc. The first stage is a CSTR corresponding to feed powder spread over a wide disc area. The second stage is a PFR corresponding to the well classified tumbling flow in the eye of the disc.
Increasing the disc angle narrows the residence time distribution and moves it towards ideal plug flow. The residence time distribution has a marked effect on granule size distribution and structure (see chapter 6).
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Figure 8.5. Residence time distributions for disc granulators (a) Experimental residence time data at disc angles between 45 and 60°; (b) Conceptual mixing model for disc granulators
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CHAPTER 8 Granulators
Drum granulators are extensively used in the mineral processing and fertiliser industries.
They are capable of very large capacity (see Table 8.1). In contrast to disc granulators, there is no size classification at the output. Drum granulators often run with large recycles of undersize and crushed oversize granules.
A granulation drum consists of an inclined cylinder which may be open ended or fitted with annular retaining rings (see figure 8.6). Feeds may be either premoistened by mixers to form granule nuclei, or liquid may be sprayed onto the tumbling bed via nozzles or distributor pipe systems. Scrapers are often, but not always, used to limit build up on the drum wall.
Figure 8.6. Typical configuration for a continuous drum granulator (Sherrington and Oliver, 1981)
Table 8.1. Characteristics of large scale granulation drums (Capes, 1980) Diameter,
*Capacity excludes recycle. Actual drum throughput may be much higher.
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TUMBLING GRANULATION 8.3.1. Granule Hold Up, Mixing and Segregation
In contrast to discs, there is no output size classification in drum granulators and high recycle rates of off-size product are common. The desired granule motion is a tumbling, cascading motion. Equation 8.1 can be used to calculate the drum critical speed with ȕ = 90. Drums are operated at a smaller fraction of critical speed than discs, typically 30 to 50% Nc. If the drum speed is too low, the granular bed will slide, rather than cascade.
High drum speeds give unwanted cataracting of the charge (similar to the motion of the balls during ball milling). Spray blow through and wall build up are more likely. Figure 8.7 illustrates different powder flow patterns in the drum.
The drum pitch angle is simply to assist the flow of the charge through the drum.
Typical drum pitch angles vary from 0 to 10°.
Hold up in the drum is between 10 and 20% of the drum volume. Holdup and mean residence time are controlled by the drum length which is typically 2 to 10 times the drum diameter. Holdup in the drum is increased by reducing the pitch angle or providing a lip at the drum exit. Residence times of 2 to 5 minutes are common.
Residence time distributions are less complex than in disc granulation. As a first approximation, granules can be considered to flow through the drum in plug flow, although some back mixing is common.
Figure 8.7. Powder flow patterns in drum granulators (a) sliding (b) cascading (c) onset of cataracting
8.3.2. Granulation Drum Circuits
Granulation circuits often involve large recycle streams (2:1 to 5:1 are common in fertiliser and iron ore circuits). The role of the recycle stream in circuit stability and control is critical. Surging and limit cycle behaviour is common. Surges alter instantaneous moisture content and size distribution in the drum (see figure 8.8). Modern control techniques with on-line moisture and size analysis can overcome these effects.
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Figure 8.8. Simulation of surging behaviour in iron ore balling circuits (Sastry, 1978)
8.4. Granulation Rate Processes
Granulation rate processes have been discussed generically in depth in part 1 of this book.
This section highlights the application of the quantitative analysis of these processes to drum and disc granulators. Readers are referred to the earlier chapters for more detailed discussion and especially to tables 3-3, 4-5 and 5-3 for trouble shooting.
The important rate processes that occur in tumbling granulators are nucleation, growth by layering or coalescence, and consolidation. Breakage and attrition are typically not important as applied stresses and impact velocities are low and there is no simultaneous drying.
Granulation processes are extremely complicated in tumbling granulators for several reasons:
• Granules remain wet and can deform and consolidate. The behaviour of a granule is therefore a function of its history.
• Different granulation behaviour is observed for broad and narrow size distribution feeds.
• There is often complex competition between growth mechanisms that may interact strongly with powder flow patterns.
8.4.1. Nucleation
Nuclei (new granules) can be formed by primary nucleation from liquid feed drops, or by secondary nucleation generated by the scraper bar. Where the liquid drop size is large compared to the feed powder size, the nuclei distribution will reflect the drop size distribution.
For drop controlled nucleation, the spray flux ȥa should be less than 0.2. To calculate ȥa (eqn.3-27), the powder surface flux is required. In a tumbling granulator, the powder surface velocity is of order of the peripheral speed of the drum wall or disc rim DN.
Provided the spray nozzles are well characterised, ȥa can be estimated without difficulty 186
TUMBLING GRANULATION
and can, in principle, be kept in the desired range by careful choice of spray nozzle(s) and liquid spray rate.
Drop controlled nucleation also requires small drop penetration times which are primarily controlled by feed formulation properties (eqn.3-27) rather than equipment parameters. As tumbling granulators have low applied stress, liquid distribution and nucleation in the mechanical dispersion regime are impossible. Thus, tumbling granulators are unsuitable, in isolation, for very fine or poorly wetting powders, highly viscous liquid binders and for mixing feeds with very different moisture contents eg. dry feed and filter cake. For these types of materials a high shear mixer granulator, or a tumbling granulator in combination with a premixer for nucleation/binder dispersion is a much better option.
In drum granulation, the flow of particles into the drum is often dominated by recycled granules. These granules are usually dried or sintered before recycle and are therefore hard and not deformable. The size range of the recycle granules is very broad, often containing particles as large as several millimetres in size. In this case, the spray drop size is often significantly smaller than many of the particles and the role of nucleation is limited. For fertiliser granulation from liquid (melt or solution) feed, the recycle represents the only particulate feed to the drum and nucleation can be neglected. This is in contrast to discs and pans, where the balance between nucleation of fine powder feed and layering of powder onto existing granules can profoundly effect the product size distribution.
8.4.2. Consolidation
Consolidation of the granules tumbling in the drum or disc is an important process. It directly determines the granule density (porosity). Consolidation can proceed over extended times and achieve high granule densities as there is no in situ drying to stop the consolidation process. Consolidation also has important indirect effects on other granulation processes by changing the amount of liquid needed to saturate the granule and changing the granulation deformability through "strain hardening".
Section 4.2 gave an extensive discussion of granule consolidation. Granule porosity decreases towards a minimum value with time in the granulator (figure 4.9):
)
The rate of consolidation is a function of the Stokes number:
)
which incorporates effects of both formulation properties and the effective collision velocity in the granulator. The minimum porosity εmin is a more complex function of binder viscosity, interparticle friction and surface forces (see section 4.2).
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reasing drum size and/or speed will increase the degree of consolidation by increasing the energy of collision. The maximum collision velocity for consolidation is proportional to the peripheral speed of the drum or disc:
2 / D
Uc ≈ω (8-2)
Thus, consolidation will increase with both drum/disc size and speed. This leads to some implications for scale up (see section 8.4.5).
8.4.3. Granule Growth by Coalescence
The fundamentals of granule growth by coalescence is covered in depth in sections 4.3 to 4.6. Coalescence is usually a key rate process in tumbling granulation. There is no simultaneous drying and relative collision velocities are fairly low, leading to a high probability of successful coalescence. Depending on the circumstances, the existing models for either near elastic collisions (section 4.3) or deformable granule collisions (sections 4.4 and 4.5) may be applicable.
In both models, the effective collision velocity is a key parameter. The relative velocity between two adjacent granules in the tumbling granulator varies with position.
The maximum collision velocity will be of order the peripheral speed of the drum or disc (eqn.8-2), and we assume that this velocity is the key to determine successful coalescence.
Thus, this single collision velocity is used to be representative of surface velocities (for nucleation calculations) and collision velocities (for consolidation and coalescence calculations).
Deformable granule coalescence
Fine powder feeds exhibit deformable granule growth in tumbling granulators. This is typically the case for disc granulators and sometimes in drum granulators. A range of growth behaviours are observed depending on the deformability of the granules (figures 8.9, 4.25). The behaviour is predictable if the Stokes deformation number (eqn.4-3) and fraction granule saturation (eqn.4-23) can be estimated. Figure 8.10 shows the growth regime map for deformable granules. The regime map has been well validated for tumbling granulators (Iveson and Litster, 1998; Iveson et al., 2001).
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TUMBLING GRANULATION
-2,000 4,000 6,000 8,000 10,000 12,000
0 100 200 300 400 500 600 700 800
Drum Revolutions
Mass Mean Diameter (m) .
11.5 wt%
10.5 wt%
10.0 wt%
9.5 wt%
Chalcopyrite with Water
Figure 8.9. Examples of growth behaviour in tumbling granulators: Induction growth behaviour of copper concentrate ( Wauters et al., 1999)
1.0E-4 1.0E-3 1.0E-2 1.0E-1 1.0E+0
0.700 0.800 0.900 1.000 1.100 1.200
Pore Saturation (-) Stdef = ρU2 /2Y (-)
Induction Nucleation Steady Growth Rapid Growth
Nucleation Region
Rapid Growth Steady
Growth
Induction
Tardos et al . (1998)
Crumb
Chalcopyrite in Drum
Ballotini & Water in Drum Iron Ore in Drum
Ballotini &
Glycerol in Drum
Figure 8.10. Growth regime map for drum granulators (Iveson et al., 2000)
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ubstantial granule growth (steady growth or induction growth) occurs within a fairly narrow range of moisture contents (85 to 110% pore saturation). Thus, an initial estimate of moisture requirements can be made by calculating the amount of liquid needed to saturate the granule pores (eqn.4-23). Capes (1980) provides a rule of thumb based on this equation:
Table 8.2 gives typical moisture requirements for a range of common materials. It is important to realise that eqn. 8-3 and Table 8.2 are only suitable for preliminary mass balance requirements for liquid binders with similar properties to water. Product granule size distributions and growth characteristics are extremely sensitive to liquid content with the narrow operating range (figures 4.29, 8.9).
For detailed discussion of the effect of other formulation properties on granule attributes, see section 4.6.
Table 8.2. Typical moisture requirements for the granulation of some common materials in tumbling granulators
Raw material
Approximate size of raw material, less than
indicated mesh
Moisture content of balled product,
wt % H2O Precipitated calcium carbonate
Hydrated lime Pulverized coal
Calcined ammonium metavanadate Lead-zinc concentrate
Iron pyrite calcine
Specular hematite concentrate Taconite concentrate
Magnetic concentrate
Direct-shipping open-pit iron ores Underground iron ore
Basic oxygen converter fume Raw cement meal
Fly ash
Fly ash-sewage sludge composite Fly ash-clay slurry composite Coal-limestone composite Coal-iron ore composite Iron ore-limestone composite Coal-iron ore-limestone composite
200